Fiber optic cables that enter buildings are commonly fitted with an isolation closure at the cable entrance of the building. The isolation closure isolates the electrically-conducting metallic elements of the fiber cable outside the building from the building itself by creating a gap in the metallic components of the cable. Most modern building codes require such an arrangement to be placed at the cable entrance to a building for protection from stray voltages and lightning. The arrangement protects systems contained within the building, such as telecom systems, computer networks and electrical power systems.
A schematic cross section of a typical optical fiber cable 200 is shown in FIG. 2. A bundle of individual optical fibers 210 is protected by a poly buffer 220 and is sealed by a water blocking tape 230. Surrounding the water blocking tape 230 is a group of strengthening wires such as piano wires 240. Those wires provide tensile strength for pulling the cable through conduit, as well as providing stiffness to prevent microbending of the fibers. The wires 240 are surrounded by a copper or stainless steel sheath 250 that provides additional protection and also provides a conductor for cable locating current used in locating the cable underground. A poly coating 260 is applied over the sheathing 250 for protection and reduced friction when pulling the cable through conduit.
A schematic illustration of an isolation closure installation 100 is shown in FIG. 1. A foundation wall 110 separates the interior of the building 112 from the ground 120. A fiber cable 125 contains a metallic portion 128, including a copper or stainless steel sheath and wire strengthening members. The cable 125 enters the interior 112 of the building through the foundation wall 110.
The poly coating is removed from a section of a cable 125, exposing a length of the metallic portion 129, 130. To isolate electrical surges traveling through the metallic portion 128 from damaging systems in the building, a roughly 6-inch gap 133 is made in the metallic portion of the cable, isolating the sections 129 and 130, and exposing the section 132 of poly buffer and possibly water blocking tape still containing the fiber bundle.
The metallic portions 129, 130 are clamped in an isolation closure 140 using clamps 135, 136. The closure is typically a cylindrical enclosure constructed of a thermoplastic or other electrically insulating material. The clamps 135, 136 and closure 140 provide rigidity to the otherwise weakened gap 133 of the fiber cable 140. Additionally, the clamp 135 may serve as a ground connection through a grounding wire 150 to ground 155. A stray power surge induced by a lightning strike or other phenomenon and traveling on the metallic portion 128 of the fiber will thereby be conducted to ground and will be isolated to prevent damage to building electrical systems.
Occasionally, maintenance or construction requires that work be done on the isolation enclosure. For example, a building may be enlarged, requiring that the isolation closure be moved to a different location on the cable, and further requiring that the section of cable formerly enclosed in the isolation closure be pulled through additional conduit. In another example, spare cable within the building must be pulled through the wall to provide additional cable length for a project on the outside of the building. In each case, the cable section that was formerly inside the isolation closure is very vulnerable when removed from the closure, because the metallic section is no longer intact to protect the fibers from microbending and from tensile loads. The cable must be protected at that point to allow it to be pulled through conduit or placed into any stressful environment. In the prior art, often the only available solution was to replace the section of cable.
There is therefore presently a need for a reliable method and apparatus for repairing the cable section enclosed in the isolation closure. Such a technique must provide protection from microbending, must provide a tensile member to absorb the tensile forced applied in pulling the cable through conduit, and must protect the components within the cable from mechanical damage and water damage. The technique should be relatively low cost, and should be easily learned by an OSP technician. To the inventors' knowledge, there is currently no such apparatus or method employed to satisfactorily accomplish that task.